1. Field of the Invention
The present invention relates to a semiconductor laser device using a gallium nitride type semiconductor and an optical information reproduction apparatus using such a semiconductor laser device. More particularly, the present invention relates to a semiconductor laser device having a desirable FFP (Far Field Pattern).
2. Description of the Related Art
Prototype semiconductor laser devices have been produced in the art using a nitride type semiconductor material, such as GaN, InN, AlN, or a mixture thereof, which emit light whose wavelength ranges from a blue region to a UV region. FIG. 16 illustrates a nitride semiconductor laser device 1600 oscillating at a wavelength of 405 nm, which was reported in Masaru KURAMOTO, et al., Jpn. J. Appl. Phys. vol. 38 (1999) pp. L184-L186. The semiconductor laser device 1600 includes an n-GaN layer 1601 (thickness: 100 xcexcm). On the n-GaN layer 1601, the semiconductor laser device 1600 further includes an n-Al0.7Ga0.93N lower cladding layer 1602 (thickness: 1 xcexcm), an n-GaN lower guide layer 1603 (thickness: 0.1 xcexcm), an In0.2Ga0.8N (thickness: 3 nm)/In0.05Ga0.95N (thickness: 5 nm)-triple quantum well active layer 1604, a p-Al0.19Ga0.81N cap layer 1605 (thickness: 20 nm), a p-GaN upper guide layer 1606 (thickness: 0.1 xcexcm), a p-Al0.07Ga0.93N upper cladding layer 1607 (thickness: 0.5 xcexcm), and a p-GaN contact layer 1608 (thickness: 0.05 xcexcm), which are deposited in this order. Electrodes 1609 and 1610 are provided on the lower side and the upper side of the device, respectively. The semiconductor laser device 1600 has a waveguide structure in which the active layer 1604 and the guide layers 1603 and 1606 are interposed between the cladding layers 1602 and 1607, so that light generated in the active layer 1604 is confined in the waveguide structure so as to cause laser oscillation.
However, the conventional semiconductor laser device 1600 has the following problems. The present inventors have produced the semiconductor laser device 1600 with the above-described structure, and obtained an FFP as shown in FIG. 17. In FIG. 17, the horizontal axis represents the angle of the beam along a plane which is perpendicular to the plane of the active layer 1604 and parallel to the longitudinal direction of the optical cavity. The vertical axis represents a relative beam intensity value. In the present specification, the term xe2x80x9cFFPxe2x80x9d refers to an FFP (i.e., an angular distribution of the light beam intensity measured at a position apart from the laser light opening of the laser device) along a direction perpendicular to the plane of the active layer. In the graph of FIG. 17, FFPs 1701 and 1702 are FFPs which have been obtained with the semiconductor laser device 1600 having the above-described structure. The FFPs 1701 and 1702 have a sub-peak in the vicinity of +20xc2x0 and have many ripples. As shown in FIG. 17, the ripples are very suppressed for some individual devices, e.g., as shown by the FFP 1701, and very significant for some other individual devices, e.g., as shown by the FFP 1702. An FFP 1703 is an FFP obtained with the semiconductor laser device 1600 in which the thickness of the n-Al0.07Ga0.93N lower cladding layer 1602 is reduced from 1 xcexcm to 0.7 xcexcm. The FFP 1703 has a very large sub-peak in the vicinity of xc2x120xc2x0.
Although not shown in FIG. 17, research by the present inventors has demonstrated that ripples, including the sub-peak in the vicinity of 20xc2x0 , are reduced by reducing the crystalline quality of the n-GaN layer 1601, which is used as a substrate, or by increasing the amount of impurity. Conversely, the ripples in the vicinity of xc2x120xc2x0 are increased when using a high quality crystal with little crystalline defect for the GaN layer 1601 and/or reducing the impurity concentration of the GaN layer 1601 in order to obtain a semiconductor laser device having a long operating life. It is believed that the differences between the ripples of FFP 1701 and those of FFP 1702 occurs due to the slight difference in terms of the conditions as described above. Moreover, it was also experimentally demonstrated that the ripples are generally more significant when the thickness of the GaN layer 1601 under the lower AlGaN cladding layer 1602 is greater. Since the thickness of the substrate is normally as great as 50 xcexcm or more, it is very difficult to suppress these ripples when using GaN as a substrate as compared to when sapphire is used as a substrate.
Thus, in the prior art, ripples occur in the FFP, and in worst cases, it is not possible to obtain a single-peak FFP intensity pattern. This can be suppressed by taking one of the above-described measures: (1) increasing the thickness of the lower cladding layer 1602: (2) reducing the crystalline quality of the GaN layer 1601; and (3) increasing the amount of impurity in the GaN layer 1601. However, if the AlGaN lower cladding layer 1602 is formed to be thick on the GaN layer 1601, as shown in (1) above, a crack may occur. If the crystalline quality of the GaN layer 1601 is reduced, as shown in (2) above, or the amount of impurity in the GaN layer 1601 is increased, as shown in (3) above, the operating life of the obtained semiconductor laser device 1600 may be reduced. Thus, these measures (1) to (3) have limited effects, and it has been difficult to adequately control the production process with a good yield.
Ripples occurring in an FFP are undesirable because they may result in insufficient focusing or generation of stray light when the device is used in an optical pickup, or the like.
According to one aspect of this invention, there is provided a semiconductor laser device, including, in this order: a GaN layer; an Alx1Ga1-x1N (0.05xe2x89xa6x1xe2x89xa60.2) lower cladding layer; an Iny1Ga1-y1N (0 less than y1 less than 1) lower guide layer (thickness: d1 [xcexcm]); an active layer (thickness: Wa [xcexcm]) having a multilayer structure comprising of alternating layers of a well layer and a barrier layer, the well layer comprising Ala1Inb1Ga1-a1-b1N1-e1-f1Pe1Asf1 (0xe2x89xa6a1, 0xe2x89xa6b1, a1+b1xe2x89xa61, 0xe2x89xa6e1, 0xe2x89xa6f1, e1+f1 less than 0.5), and the barrier layer comprising Ala2Inb2Ga1-a2-b2N1-e2-f2P2eAsf2 (0xe2x89xa6a2, 0xe2x89xa6b2, a2+b2xe2x89xa61, 0xe2x89xa6e2, 0xe2x89xa6f2, e2+f2 less than 0.5); an Iny2Ga1-y2N (0 less than y2 less than 1) upper guide layer (thickness: d2 [xcexcm]); and an Alx2Ga1-x2N (0.05xe2x89xa6x2xe2x89xa60.2) upper cladding layer, wherein: the thicknesses and the compositions of the lower guide layer and the upper guide layer are set such that ripples in a far field pattern in a direction perpendicular to a stack plane are suppressed.
According to another aspect of this invention, there is provided a semiconductor laser device, including, in this order: a GaN layer; an Alx1Ga1-x1N (0.05xe2x89xa6x1xe2x89xa60.2) lower cladding layer; an Iny1Ga1-y1N (0 less than y1 less than 1) lower guide layer (thickness: d1 [xcexcm]); an active layer (thickness: Wa [xcexcm]) having a multilayer structure comprising of alternating layers of a well layer and a barrier layer, the well layer comprising Ala1Inb1Ga1-a1-b1N1-a1-f1Pa1Asf1 (0xe2x89xa6a1, 0xe2x89xa6b1, a1+b1xe2x89xa61, 0xe2x89xa6e1, 0xe2x89xa6f1, e1+f1 less than 0.5), and the barrier layer comprising Ala2Inb2Ga1-a2-b2N1-e2-f2Pe2Asf2 (0xe2x89xa6a2, 0xe2x89xa6b2, a2+b2 less than 1, 0xe2x89xa6e2, 0xe2x89xa6f2, e2+f2 less than 0.5); an Iny2Ga1-y2N(0 less than y2 less than 1) upper guide layer (thickness: d2 [xcexcm]); and an Alx2Ga1-x2N (0.05xe2x89xa6x2xe2x89xa60.2) upper cladding layer, wherein: the thicknesses and the compositions of the lower guide layer and the upper guide layer are set such that an oscillating mode effective refractive index neq of oscillation light from the semiconductor laser device and a refractive index nGaN of the GaN layer have a relationship of neqxe2x89xa7nGaN.
According to still another aspect of this invention, there is provided a semiconductor laser device, including, in this order: a GaN layer; an Alx1Ga1-x1N (0.05xe2x89xa6x1xe2x89xa60.2) lower cladding layer; an Iny1Ga1-y1N (0 less than y1 less than 1) lower guide layer (thickness: d1 [xcexcm]); an active layer (thickness: Wa [xcexcm]) having a multilayer structure comprising of alternating layers of a well layer and a barrier layer, the well layer comprising Als1Inb1Ga1-a1-b1N1-e1-f1Ps1Asf1 (0xe2x89xa6a1, 0xe2x89xa6b1, a1+b1xe2x89xa61, 0xe2x89xa6e1, 0xe2x89xa6f1, e1+f1 less than 0.5), and the barrier layer comprising Ala2Inb2Ga1-a2-b2N1-e2-f2Pe2Asf2 (0xe2x89xa6a2, 0xe2x89xa6b2, a2+b2xe2x89xa61, 0xe2x89xa6e2, 0xe2x89xa6f2, e2+f2 less than 0.5); an Iny2Ga1-y2N (0 less than y2 less than 1) upper guide layer (thickness: d2 [xcexcm]); and an Alx2Ga1-x2N (0.05xe2x89xa6x2xe2x89xa60.2) upper cladding layer, wherein: the thicknesses and the compositions of the lower guide layer and the upper guide layer are set so as to satisfy one of the following relationships:
0.06xe2x89xa6d1+d2xe2x89xa60.1 and 0.06xe2x89xa6y1, 0.06xe2x89xa6y2;
0.1 less than d1+d2xe2x89xa60.15 and 0.04xe2x89xa6y1, 0.04xe2x89xa6y2;
0.15 less than d1+d2xe2x89xa60.2 and 0.03xe2x89xa6y1, 0.03xe2x89xa6y2;
0.2 less than d1+d2xe2x89xa60.3 and 0.015xe2x89xa6y1, 0.015xe2x89xa6y2; and
0.3 less than d1+d2 and 0.01xe2x89xa6y1, 0.01xe2x89xa6y2.
According to still another aspect of this invention, there is provided a semiconductor laser device, including, in this order: a GaN layer; an Alx1Ga1-x1N (0.05xe2x89xa6x1xe2x89xa60.2) lower cladding layer; an Iny1Ga1-y1N (0 less than y1 less than 1) lower guide layer (thickness: d1 [xcexcm]); an active layer (thickness: Wa [xcexcm]) having a multilayer structure comprising of alternating layers of a well layer and a barrier layer, the well layer comprising Als1Inb1Ga1-a1-b1N1-e1-f1Pe1Asf1 (0xe2x89xa6a1, 0xe2x89xa6b1, a1+b1xe2x89xa61, 0xe2x89xa6e1, 0xe2x89xa6f1, e1+f1 less than 0.5), and the barrier layer comprising Ala2Inb2Ga1-a2-b2N1-e2-f2Pe2Asf2 (0xe2x89xa6a2, 0xe2x89xa6b2, a2+b2 less than 1, 0xe2x89xa6e2, 0xe2x89xa6f2, e2+f2 less than 0.5); an Iny2Ga1-y2N (0 less than y2 less than 1) upper guide layer (thickness: d2 [xcexcm]); and an Alx2Ga1-x2N (0.05xe2x89xa6x2xe2x89xa60.2) upper cladding layer, wherein: the thicknesses and the compositions of the lower guide layer and the upper guide layer are set so as to satisfy the following relationships:
yxe2x89xa70.003/dxe2x88x920.003+(0.007xe2x88x920.22xc3x97Wa)+(xe2x88x920.010+0.10xx)
[where
d=(d1+d2)/2,
y=(y1xc3x97d1+y2xc3x97d2)/(d1+d2),
x=(x1+x2)/2].
According to still another aspect of this invention, there is provided an optical information reproduction apparatus for reproducing information recorded on an optical disk having an information recording surface by irradiating the optical disk with laser light and photoelectrically converting the laser light reflected from the optical disk, wherein the optical information reproduction apparatus uses a semiconductor laser device of the present invention as a light source.
According to still another aspect of this invention, there is provided an optical information reproduction apparatus for reproducing information recorded on an optical disk having an information recording surface by irradiating the optical disk with laser light and photoelectrically converting the laser light reflected from the optical disk, wherein the optical information reproduction apparatus uses a semiconductor laser device of the present invention as a light source.
According to still another aspect of this invention, there is provided an optical information reproduction apparatus for reproducing information recorded on an optical disk having an information recording surface by irradiating the optical disk with laser light and photoelectrically converting the laser light reflected from the optical disk, wherein the optical information reproduction apparatus uses a semiconductor laser device of the present invention as a light source.
According to still another aspect of this invention, there is provided an optical information reproduction apparatus for reproducing information recorded on an optical disk having an information recording surface by irradiating the optical disk with laser light and photoelectrically converting the laser light reflected from the optical disk, wherein the optical information reproduction apparatus uses a semiconductor laser device of the present invention as a light source.
According to still another aspect of this invention, there is provided a semiconductor laser device, including: a GaN layer; an Alx1Ga1-x1N (0.05xe2x89xa6x1xe2x89xa60.2) lower cladding layer; an Iny1Ga1-y1N (0 less than y1 less than 1) lower guide layer; an active layer having a multilayer structure comprising of alternating layers of a well layer and a barrier layer, the well layer comprising Ala1Inb1Ga1-a1-b1N1-e1-f1Pe1Asf1 (0xe2x89xa6a1, 0xe2x89xa6b1, a1+b1xe2x89xa61, 0xe2x89xa6e1, 0xe2x89xa6f1, e1+f1 less than 0.5), and the barrier layer comprising Ala2Inb2Ga1-a2-b2N1-e2-f2Pe2Asf2 (0xe2x89xa6a2, 0xe2x89xa6b2, a2+b2xe2x89xa61, 0xe2x89xa6e2, 0xe2x89xa6f2, e2+f2 less than 0.5); and Iny2Ga1-y2N (0 less than y2 less than 1) upper guide layer; and an Alx2Ga1-x2N (0.05xe2x89xa6x2xe2x89xa60.2) upper cladding layer, wherein: the thickness d1 xcexcm of the lower guide layer, the In composition y1 of the lower guide layer, the thickness d2 xcexcm of the upper guide layer, and the In composition y2 of the upper guide layer satisfy the following relationships:
0.06xe2x89xa6d1+d2,
0.01xe2x89xa6y1, and 0.01xe2x89xa6y2.
In one embodiment of the invention, the thickness d1 xcexcm of the lower guide layer and the In composition y1 of the lower guide layer satisfy the following relationship: y1xe2x89xa60.003/d1xe2x88x920.003.
In one embodiment of the invention, the thickness d1 xcexcm of the lower guide layer and the In composition y1 of the lower guide layer satisfy the following relationship: y1xe2x89xa70.003/d1+0.002.
In one embodiment of the invention, the thickness d2 xcexcm of the upper guide layer and the In composition y2 of the upper guide layer satisfy the following relationship: y2xe2x89xa70.003/d2xe2x88x920.003.
In one embodiment of the invention, the thickness d2 xcexcm of the upper guide layer and the In composition y2 of the upper guide layer satisfy the following relationship: y2xe2x89xa70.003/d2+0.002.
In one embodiment of the invention, the In composition y1 of the lower guide layer, the thickness d1 xcexcm of the lower guide layer, the In composition y2 of the upper guide layer, the thickness d2 xcexcm of the upper guide layer and the thickness Wa xcexcm of the active layer satisfy the following relationship:
yxe2x89xa70.003/dxe2x88x920.003+(0.007xe2x88x920.22xc3x97Wa)
where
d=(d1+d2)/2, and
y=(y1xc3x97d1+y2xc3x97d2)/(d1+d2).
In one embodiment of the invention, the In composition y1 of the lower guide layer, the thickness d1 xcexcm of the lower guide layer, the In composition y2 of the upper guide layer, the thickness d2 xcexcm of the upper guide layer and the thickness Wa xcexcm of the active layer satisfy the following relationship:
yxe2x89xa70.003/d+0.002+(0.007xe2x88x920.22xc3x97Wa)
where
d=(d1+d2)/2, and
y=(y1xc3x97d1+y2xc3x97d2)/(d1+d2).
In one embodiment of the invention, the In composition y1 of the lower guide layer, the thickness d1 of the lower guide layer, the Al composition y2 of the upper guide layer, the thickness d2 of the upper guide layer and the Al composition x2 of the upper cladding layer satisfy the following relationship:
yxe2x89xa70.003/dxe2x88x920.003+(xe2x88x920.010+0.10xx)
where
d=(d1+d2)/2,
y=(y1xc3x97d1+y2xc3x97d2)/(d1+d2), and
x=(x1+x2)/2.
In one embodiment of the invention, the In composition y1 of the lower guide layer, the thickness d1 of the lower guide layer, Al composition x1 of the lower cladding layer, the In composition y2 of the upper guide layer, the thickness d2 of the upper guide layer and the Al composition x2 of the upper cladding layer satisfy the following relationship:
yxe2x89xa70.003/d+0.002+(xe2x88x920.010+0.10xx)
where
d=(d1+d2)/2,
y=(y1xc3x97d1+y2xc3x97d2)/(d1+d2), and
x=(x1+x2)/2.
According to still another aspect of this invention, there is provided an optical information reproduction apparatus, including: a semiconductor laser device of the present invention; and a photodetector, wherein: laser light is emitted from the semiconductor laser device to irradiate an optical disk, and information recorded on the optical disk is reproduced based on the laser light reflected from the optical disk.
Thus, the invention described herein makes possible the advantages of: (1) eliminating the above-described problems and providing a nitride semiconductor laser device which is optimally used in an optical pickup; and (2) providing an optical information reproduction apparatus having a good focusing characteristic.